MST BPDUs on 6500s with spanning tree disabled

Unanswered Question
Jul 28th, 2008


I have a question regarding MST BPDUs and how switches with spanning tree disabled treat them? Here is the rough scenario

mst root(6509)--------------mst sec(6509)

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| |

| |

6509(spantree disabled)-------6509(spantree disabled)

BPDUs from the MST switches are making to the access 6509s (with spantree disabled) then disapearing. Now I think the switches with Spanning tree disabled should just forward the BPDUs transparently but, I obviously could be wrong here.

So the Question is do cisco switches with spanning tree disabled forward BPDUs or are they discarded by default?

(the four switches are connected in a "square" fashion)

Many thanks.

I have this problem too.
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Giuseppe Larosa Tue, 07/29/2008 - 07:38

Hello Olumide,

MST BPDUs are sent to the route processor and there are discarded if STP disabled or processed if necessary(when STP is enabled).

Think that with ethertype ACLs on several platforms filtering BPDUs is not allowed.

This is a better behaviour then letting them to go through the switch.

I wonder if with MST you can really disable spanning tree. There is the infrastructure STP instance that could be running

However, disabling STP is not recommended and there are no reasons to do it on powerful devices like C6509.

So I would suggest to avoid to do this on a production network.

To have STP BDUs tunneled to another site you should use 802.1Q tunneling (Q in Q) and add some specific commands

Hope to help


olumidekomolafe Fri, 08/01/2008 - 06:33

Many thanks for that Giuseppe..

I definitely agree that diabling STP is not recommended.


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